Various types of gauging systems are used to determine the height of a liquid level within storage tanks. These include mechanical systems (e.g., flotation devices), ultrasonic and optical systems, among others. In the storage of highly volatile fluids such as liquid natural gas (LNG), there is a particular need for a gauging system which is extremely accurate, reliable and requires minimal maintenance, because of the many safety problems and costs associated with opening the tank for access. An especially suitable system for such applications is one of the type disclosed in U.S. Pat. Nos. 3,301,056, 3,533,286, and 3,797,311. In such a system, a capacitive sensor extending along the height of the tank is oriented such that the liquid rises between its electrodes, the capacitance of the sensor being dependent on the height of the liquid between the electrodes. The sensor capacitance, as measured by an associated electronic control unit, is compared with that of a similarly constructed reference sensor which is totally immersed in the liquid, to yield an indication of the liquid height.
Often, when LNG is transferred from one tank to another, for example when being off-loaded from a shipboard tank to a shore installation or vice versa, a transfer of ownership occurs, involving an exchange of money. Since typically millions of cubic feet of LNG are involved, a measurement error of less than a percent can result in a significant error in the cost of the transaction. Therefore it is desirable to calibrate the level gauging system at specified times and after any repair of the electronics.
A typical calibration based on zero and full-scale values of sensor capacitance begins with the establishment of a reference baseline indicative of the sensor when it is totally drained of fluid. In operation, to insure an accurate determination of the liquid level, this baseline must be: (1) subtracted in some way from the measured capacitance value of the sensor, and (2) employed in some way to establish a system scale factor. Unfortunately, at the time of transfer, both the delivering tank and the receiving tank are usually partially filled; in fact, it is likely that a shore receiving tank may never be emptied. So, a direct measurement of either tank's sensor in an empty condition is usually impossible.
Until now, the calibration was performed by uncoupling from the electronic control unit the cables interconnecting the sensors to the control unit, and substituting in their place adjustable precision capacitors set to the laboratory measured "empty" values of each sensor. However, this procedure does not take into account the electrical effects of the interconnecting cables on the measurement system. In the case of a shipboard tank installation, these cables may be on the order of 250 meters in length, and for shore-based installations, up to 750 meters or so. When coupled between its sensor and the control unit, a cable introduces additional capacitance and other impedance and transmission-related effects, which produce a different net effect at the control unit than does the capacitance value of the sensor by itself. The voltage at the end of the cable differs both in magnitude an phase from the voltage at the sensor. If these effects are not accounted for, the accuracy of the calibration, and consequently of the measurement, is compromised.
Another drawback of the conventional method of calibration occurs in the case of sensors which experience abnormally rough handling in shipment or installation. In such cases the sensor capacitance may deviate from the original laboratory value. When used in rugged environments such as shipboard tanks, the sensors may undergo further small changes in capacitance over the life of the system. In this way, small errors may be introduced when calibrating the system conventionally in accordance with original laboratory measured data.
Yet another drawback of the conventional method of calibration is that the capacitive sensor often is made up of several segments, and in order to simultaneously simulate the entire sensor, several precision capacitors are needed, one for each segment. However, these capacitors tend to be bulky and quite expensive, which further complicates the procedure.
Therefore, it is an object of the present invention to provide a calibrator which can accurately duplicate the total electrical effects of a capacitive sensor and its associated cable under a predetermined set of conditions.
It is a further object of the present invention to provide such a calibrator which can interface easily with existing tank gauging systems.